This invention relates to amplifier switching, for example for switching of low noise amplifiers in receive paths of wireless communications systems.
It is well known in communications systems to use a single antenna for both transmitting and receiving signals, a duplexer being provided to couple an output of a transmit power amplifier to the antenna and to couple the antenna to a receive signal path, while isolating the receive path from the output of the transmit power amplifier.
For example, in a base station of a cellular wireless communications system the duplexer may comprise cavity filters for filtering separate transmit and receive signal bands. The receive path includes a receive path filter of the duplexer and a low noise amplifier (LNA) for amplifying the receive path signal. These units are required to provide low insertion lose (high return loss) and good noise figure, these being significant parameters for operation of the system, and the LNA is required to have good linearity. For example, the receive path filter of the duplexer has a high cost to achieve a desirably low insertion loss of the order of 0.4 dB.
In different arrangements of such a system, the duplexer may or may not itself include a LNA. If it does not, then the LNA is provided subsequently in the receive path. However, if the duplexer itself includes a LNA (for example the duplexer may be provided at the antenna so that amplification of the receive path signal is desirable before it is communicated over a relatively long signal path, or a tower-mounted LNA may be equivalently provided), then it is desirable for the gain provided by such a subsequent go to be reduced or eliminated, in order to avoid non-linearity of this subsequent LNA as a result of the relatively high receive signal level which it receives from the duplexer LNA. Also, and for similar reasons, it may be desirable to provide gain switching of one or more LNAs which may be cascaded in the receive signal path, or generally to extend the dynamic gain range of one or more LNAs.
It is therefore desirable to provide switching, for example to bypass the subsequent LNA when its amplification is not required. However, introducing a switch (for example, a MESFET switch) into the receive signal path, selectively to bypass the receive signal around the LNA, increases the noise figure by, for example, about 0.5 dB. While this in not a problem when the receive path signal has already been amplified, it represents a significant increase to the noise figure of the LNA, typically about 0.7 dB, for a receive path signal which has not been previously amplified.
In addition, in a balanced LNA arrangement which is desirable to provide high return loss and hence low loading of the duplexer receive path filter, using only a single switch in the receive signal path results in the return lose of the overall LNA arrangement being reduced significantly to that of the switch; for example it may be reduced from better than 20 dB to only 14 dB for the switch.
A switched bypass can alternatively be provided around only a second gain stage of such a subsequent LNA having first and second gain stages. Whilst this substantially avoids the noise figure increase discussed above, it does not overcome the-potential for consequent non-linearity of the first gain stage of the LNA. Similarly, AGC (automatic gain control) arrangements are not effective to provide a desirable amount of receive path gain for both low noise figure and linear operation of the LNA arrangement.
Accordingly, there is a need to provide an improved arrangement for amplifier switching.
According to one aspect of this invention there is provided an amplifier arrangement comprising: an amplifier having an input and an output; a coupler having at least a first port for an input signal, a second port coupled to the input of the amplifier for supplying the input signal thereto, and a third port for a component of the input signal reflected back to the second port from the input of the amplifier; a switch for coupling the third port to a load in a first state of the switch and to a bypass path in a second state of the switch; and an output of the amplifier arrangement derived from the output of the amplifier in the first state of the switch and from the bypass path in the second state of the switch.
The arrangement preferably includes a switch for interrupting a supply voltage of the amplifier, and/or a switch for shunting the input of the amplifier to ground, in said second state of the first switch, to increase mismatch and hence signal reflection at the input of the amplifier.
The arrangement can include a switch for coupling the output of the amplifier arrangement to the output of the amplifier in said first state of the first switch and to the bypass path in said second state of the first switch, or it can include a signal combiner for coupling the output of the amplifier and the bypass path to the output of the amplifier arrangement.
The arrangement is desirably a balanced amplifier arrangement which further comprises a second amplifier having an input and an output, the input of the second amplifier being coupled to a fourth port of the first coupler; and a second coupler for coupling the outputs of the amplifiers to the output of the amplifier arrangement.
According to another aspect, the invention provides a balanced amplifier arrangement comprising: two amplifiers each having an input and an output; a first coupler having a first port for an input signal, second and third ports coupled to the inputs of the amplifiers, and a fourth port, the coupler being such that an input signal supplied to the first port is coupled to the second port and to the third port with a relative phase difference of 90xc2x0, and components of the input signal reflected back to the first coupler from the inputs of the amplifiers cancel at the first port and add at the fourth port of the coupler; a second coupler for combining signals at the outputs of the amplifiers in phase with one another; a switch for coupling the fourth port of the first coupler to a load in a first state of the switch and to a bypass path in a second state of the switch; and an output of the amplifier arrangement derived from the combined outputs of the amplifiers in the first state of the switch and from the bypass path in the second state of the switch.
This arrangement preferably includes a switch for interrupting a supply voltage of the amplifiers, and/or two switches for shunting the inputs of the amplifiers to ground, in said second state of the first switch, to increase mismatch and hence signal reflection at the inputs of the amplifiers.
The arrangement can include a switch for coupling the combined outputs of the amplifiers to the output of the amplifier arrangement in said first state of the first switch and for coupling the bypass path to the output of the amplifier arrangement in said second state of the first switch, or it can include a signal combiner for coupling the combined outputs of the amplifies and the bypass path to the output of the amplifier arrangement.
In one form of the balanced amplifier arrangement, the second coupler comprises a phase quadrature coupler having two ports coupled to the outputs of the amplifiers, a third port coupled to the output of the amplifier arrangement, and a fourth port, the arrangement further comprising a further switch for coupling the fourth port of the second coupler to a load in said first state of the first switch and to the bypass path in said second state of the first switch.
The invention also provides a duplexer arrangement for a communications system, the duplexer arrangement comprising an amplifier arrangement or a balanced amplifier arrangement as recited above, a transmit path for coupling a signal from an output of a transmitter to an antenna, and a receive path including a receive filter for coupling a received signal from the antenna to said first port of the (first) coupler of the amplifier arrangement as said input signal.
The invention further provides a method of selectively bypassing an amplifier in an amplifier arrangement, comprising the steps of: supplying a signal to an input of the amplifier; when the amplifier is not bypassed, dissipating in a load components of the signal which are reflected at the input of the amplifier; and selecting components of the signal which are reflected at the input of the amplifier to bypass the amplifier.
Preferably the method includes the step of increasing signal reflection at the input of the amplifier when said components are selected to bypass the amplifier. The step of increasing signal reflection at the input of the amplifier conveniently comprises interrupting a supply voltage to the amplifier and/or shunting the input of the amplifier to ground.